1. Field of the Invention
This invention relates to circuit breakers, and more particularly to an improved design for a support plate and its mounting in a base of a housing of the circuit breaker.
2. Background Information
A common type of circuit breaker used to automatically interrupt abnormal currents in an electrical system incorporates a thermal trip device which responds to persistent low levels of overcurrent and a magnetic trip assembly which responds instantly to higher levels of overcurrent. In such circuit breakers the thermal trip device comprises a bimetal which flexes in response to the persistent low level overcurrent passed through it to unlatch a latchable operating mechanism. The latchable operating mechanism is spring operated to open electrical contacts which interrupt the current. Typically, the circuit breaker mechanism is mounted in a housing comprising a base section forming a cavity in which the circuit breaker mechanism is assembled, and a cover which is secured in place over the base to enclose the circuit breaker mechanism. Industry standards require that the thermal trip device in these circuit breakers be calibrated to trip the breaker in response to an overcurrent of a predetermined magnitude within a specified time interval. Commonly, this calibration of the thermal trip is performed "on the half shell." That is, the circuit breaker mechanism is assembled within the cavity of the breaker housing, and the thermal trip is calibrated before the mechanism is enclosed by the cover.
A common type of circuit breaker in which the thermal trip is calibrated in this manner is shown by way of example in U.S. Pat. No. 3,849,747. Such circuit breakers have been in use for many years and their design has been refined to provide an effective, reliable circuit breaker which can be easily and economically manufactured on a large scale. This type of circuit breaker has a metal support plate with an integral tab extending laterally from one end to which the bimetal of the thermal trip device is secured. The end of the support plate from which the tab extends is partially separated from the remainder of the support plate which is fixed in the housing by a transverse slot. The bimetal is calibrated by closing the circuit breaker and applying the prescribed overcurrent. A tool is inserted in the transverse slot in the support plate and when the specified time has expired, the tool is rotated to distort the free end of the support plate thereby adjusting the position of the support for the bimetal to cause the bimetal to trip the breaker. This calibration is presently carried out automatically, "on the half shell" by a machine. With the calibration set, the cover is installed and riveted in place. The circuit breaker is then tested to validate the calibration. Circuit breakers which do not pass the calibration test are reworked by inserting a hook through a slot in the end of the circuit breaker to engage the free end of the bimetal to attempt to bring it within tolerance. Such reworking is done manually, and being difficult to perform, only results in bringing about half of the rejected circuit breakers into tolerance.
It has been determined that the number of circuit breakers which fail the calibration test performed after the cover has been installed is in part due to minor changes in position and distortion of the mechanism resulting from misalignment of the housing pans causing the breaker to fall out of calibration. In order to overcome these effects, U.S. Pat. No. 4,148,004 proposes a circuit breaker of this type which is fully assembled with the cover riveted in place, and is then calibrated by a plug rotatably mounted in the wall of the housing and having a bifurcated stem which engages the tab on the support plate and the fixed end of the bimetal. A tool is inserted in apertures in the external face of the calibrating plug and rotated to set the calibration. Thus, the circuit breaker is calibrated after it has been fully assembled and the parts are fixed in their final position. However, it also allows one to change the calibration which is not in conformance with electrical codes in the United States.
U.S. Pat. No. 5,008,645 proposes a circuit breaker which overcomes the shortcomings of U.S. Pat. No. 4,148,004 which provides an indication that an attempt has been made to change the calibration once it has been set. In a preferred embodiment of the invention of U.S. Pat. No. 5,008,645, the circuit breaker assembly includes a support plate mounted in a cavity in the circuit breaker housing and extending along a planar wall of the housing. This support plate has a main portion fixed in the housing and a free end partially separated from the main portion by a transverse slot. The free end of the support plate supports the bimetal of the trip assembly. The calibration opening extends through the planar wall of the base of the housing and is aligned with the transverse slot in the support plate through which a tool is inserted to engage the transverse slot and rotate the free end of the support plate carrying the bimetal to calibrate the circuit breaker at the selected persistent current overload with the circuit breaker assembled and enclosed within the housing. The calibration opening of this embodiment of the invention is provided with tamper indicating seal means.
The support plate of this U.S. Pat. No. 5,008,645 has a length such that it substantially extends along the width of the planar wall of the base of the housing with the main portion being fixed in the housing by way of an opening which is keyed to and engaged by a projection of a cradle support post molded into the planar wall of the housing base. A further opening which is oval-shaped and located near the transverse slot fits snugly over a pin molded on the planar wall of the base, and cooperates with the opening of the main portion and its mounting on the cradle support post to firmly fix the position of the support plate within the housing base.
In this U.S. Pat. No. 5,008,645 the anchoring of the support plate onto the cradle support post to firmly fix the support plate into position was necessary in that the material for the base and the cover of the circuit breaker was subjected to changes in the temperature causing the circuit breaker components to expand or contract, including the support plate, which resulted in a disturbance of the calibration setting. Since the base and cover are now being made of a fiberglass material which is a better quality plastic material which does not contract and/or expand during the changes in temperature in the circuit breaker, it is not necessary to provide a support plate extending the length of the planar wall of the base for its mounting on the cradle support post. The support plate in a base made of fiberglass need only support the bimetal of a trip mechanism.
From the above, it can be appreciated that a support plate is necessary for calibration purposes. The demands of the electrical industry are for smaller and cheaper circuit breakers, and higher current interruption ratings. These criteria can be realized to some extent in view of the base and cover now being made of a glass polyester material such as fiberglass, which is dimensionally a more stable and a much stronger kind of plastic than previously available, and which does not contract and/or expand due to temperature fluctuations. Another factor which may help realize these criteria are smaller components for the circuit breaker assembly.
The circuit breaker which meets these criteria may still require a support plate for supporting a bimetal of a trip mechanism which is bent for calibration purposes.
There remains, therefore, a need for a circuit breaker which is cheaper to manufacture, which is smaller and more compact, and which has a higher interruption current rating than present-day circuit breakers.
More particularly, since the circuit breakers will be required to be smaller and thinner there is a need for such a circuit breaker to have a smaller and more shallow cavity for supporting the circuit breaker assembly than the cavity of present-day circuit breakers and, therefore, a need for an improved design for a support plate used for calibration purposes and for supporting the bimetal of the trip mechanism.
These and other needs are satisfied by the present invention.